1. Field of the Invention
The present invention relates to a flash memory device. For example, the present invention relates to a control system for an SD™ memory card using a NAND flash memory.
2. Description of the Related Art
For example, a NAND flash memory used for an SD memory has the following problem. Specifically, the same address data is repeatedly read at a very high frequency, and thereby, there exists a phenomenon (read disturbance [RD]) such that data stored at the address is destroyed.
For example, according to a file allocation table (FAT) system, management information such as FAT and directory entry are repeatedly read when data is accessed depending on host apparatus characteristics.
Specifically, every time cluster data of a file is read, a FAT corresponding to the cluster is frequently read at a small data size (e.g., 512 bytes). For example, according to a FAT16 having a cluster size of 16 Kbytes, a 512-byte FAT area has 256 FAT data. If a 4 Mbytes (256 clusters) file is assigned to a continuous cluster, when the corresponding FAT data is read every time one cluster data is read, the 512-byte FAT area data is read 256 times while a file is read. If the foregoing read operation is carried out with respect to the same page of a NAND flash memory, the number of page reads becomes great; as a result, the risk of generating RD increases. In other words, in the NAND flash memory, data read/write are carried out at a page unit. For this reason, if the 512-byte FAT area data is stored in one page, the same page is accessed 256 times.
A file entry is a 32-byte data block stored with a file name to a file, a start cluster address of the file and a file size. In the file entry, the same data is repeatedly read every time the file is accessed. Therefore, if the file entry is read from the same page of the NAND flash memory, the number of page reads becomes great; as a result, the risk of generating RD increases. Accordingly, it is desired to provide a flash memory device capable of effectively preventing data destruction by read disturbance.
It should be noted that, although a technique of preventing RD is not provided, a hard disk file management system has been known as the related art (e.g., see Jpn. Pat. Appln. KOAKI Publication No. 11-134255).